Parabolic as well as other circularly shaped reflectors are typically used in satellite communications for receiving radio signals from or transmitting radio signals to satellites. When receiving signals, the reflectors are used to gather incoming signals and then reflect them to a region, designated as the focal point, where the signals are concentrated and received by the antenna feed. Reciprocally, when transmitting signals, the signals are transmitted first through the antenna feed, after which, the signals disperse in such a manner as to illuminate the reflector, after which, the signals are directed by the reflector towards a satellite or other target. For simplicity the following discussion will be in terms of the antenna used to receive signals.
Because of the heavy demand for signal bandwidth, it has become a practice in the industry, in particular satellite to ground transmissions, to double the available bandwidth by simultaneously transmitting orthogonal signals. This can be accomplished by transmitting electric fields, which are linearly polarized as would be the case for a first bandwidth, which is vertically polarized, and a second bandwidth, which is horizontally polarized. Likewise, the two bandwidths can be elliptically or circularly polarized with a first bandwidth with clockwise polarization and a second bandwidth with counter-clockwise polarization.
A waveguide feed operating as an antenna feed commonly consists of a waveguide cavity to gather the signals which have been concentrated by the reflector and a probe or probes to couple the signals from the cavity to a circuit board which will usually have electronic devices to amplify and process the signals.
In many instances, as in the case for a coaxial waveguide feed, two frequencies are supplied to a single feed through the use of two probes aligned with the electrical fields of orthogonal TE.sub.11 coaxial waveguide modes. However, care should be taken to ensure that the probes are not aligned with the electric fields of the TEM mode. By coupling to the TEM mode the probes couple to each other, which causes them to have poor polarization isolation. The alignment of the probes needs to be such that they excite the primary coaxial waveguide mode, TE.sub.11, while suppressing the excitation of the TEM or coaxial mode.
In order for a single probe in circular shaped coaxial waveguide to be perfectly orthogonal to the TEM mode it would have to be circular in shape with perfect symmetry. There is no such shape that can also be used to excite the TE.sub.11 mode. A common method is to use multiple probes or probes oriented in a complementary fashion. If complementary probes are excited with equal amounts of power but with a 180.degree. phase difference, the TE.sub.11 waveguide mode will be excited with no excitation of the TEM mode. A similar excitation of orthogonal complementary probes will likewise excite a second but orthogonal TE.sub.11 mode with no excitation of the TEM mode. Such a scheme is complicated and, when receiving weak satellite signals, the necessary higher losses results in higher noise figures.
Many of the newer satellites transmit multiple bandwidths, which vary considerably in frequency. A good example is the use of the C-Band (typically 3.7 GHz to 4.2 GHz) and the Ku-Band (typically 11.7 GHz to 12.7 GHz) bandwidths. With approximately a 3:1 frequency ratio between the bandwidths, both bandwidths can be simultaneously received by waveguide cavities that are co-located. This is accomplished by placing the Ku-Band waveguide cavity within and with its axis aligned with the C-Band waveguide, which has a cross-sectional dimension of approximately three times the size of the Ku-Band waveguide.
While this co-location of waveguide cavities reduces the overall area required by the antenna feed, it is still rather large and cumbersome since each of the waveguide cavities is constructed as an individual or stand-alone cavity and they are then simply placed in axial alignment.
Accordingly, it would be desirable to provide a waveguide feed which overcomes this drawback.
It is an object of the present invention to provide a new and improved waveguide feed.
It is another object of the present invention to provide a new and improved waveguide feed with reduced external dimensions.
It is still another object of the present invention to provide a new and improved waveguide feed with reduced external dimensions which provides improved isolation and VSWR.
It is a further object of the present invention to provide a new and improved waveguide feed with a coaxial waveguide having a reduced outer dimension.